Electrically operated mode switching mechanisms are often used in tape recorders in place of conventional mechanically controlled mode switching mechanisms. Since a mode switching mechanism of the electrically operated type permits selection of a desired mode in response to a featherly touch on the associated key or button, it has gained wide popularity among users. However, the conventional electrically operated mode switching mechanism employs a plurality of solenoids which are associated with different operating keys. This requires a substantial amount of power for operating various mechanical elements particularly because the circuitry is designed so that the solenoids remain energized during the time the recorder is being operated. Although this continuous power operation may find use in applications where the power is supplied from an external A.C. source, application to battery powered portable tape recorders is not possible due to its power consumption. Furthermore, the use of a plurality of solenoids results in a relatively large size and a high manufacturing cost.
The inventor of the present invention divised, prior to the present invention, a pulse-operated mode switching mechanism for tape recorders and filed patent applications Ser. No. 54-38006 in Japan, 134,990 in the United States, now U.S. Pat. No. 4,336,560, 8010392 in United Kingdom, and P 3012196.1 in West Germany. According to the technique of the prior applications by the present inventor, a pulse operated mode switching mechanism for tape recorders comprises a single solenoid which is operated in response to a pusle of different periods depending on the function of the operating key. A rotary control wheel is driven by a motor to provide a 360 degree revolution in response to the energization of the solenoid. Electromagnetic transducer head(s) and an erase head are mounted on a carrier plate which is movable between a first position in which the heads are remote from the magnetic tape and a second position in which the tape is in cotact with the heads. A pinch roller or idler is arranged to be biased toward the capstan to drive the tape when the carrier plate assumes the first position. The carrier plate is moved from the first to second position by the rotation of the rotary control wheel when the solenoid is energized for an interval greater than a first predetermined interval so that playback or recording is performed. Energization of the solenoid for an interval smaller than a second predetermined interval, which is smaller than the first predetermined interval, during playback or recording mode causes the carrier plate to return to the first position so that movement of the tape terminates to assume stop mode. On the other hand, when the solenoid is energized for an interval greater than the second interval but smaller than the first interval, the carrier plate is arranged to be locked at an intermediate position between the first and second positions causing the idler to be remote from the capstan, resulting in pause operation. Namely, in accordance with the technique of the prior invention of the present inventor, the position of the carrier plate is controlled by the rotation of the control wheel in response to the operation of the solenoid, where the duration of the pulse applied to the solenoid determines the mode to be assumed. To this end one of three pulses having different intervals is selected in accordance with a selected key, such as playback key, record key, stop key, fast forward key, rewind key, and pause key. The three different intervals are all predetermined so that desired operations are respectively performed.
However, the above-described pulse-operated mode switching mechanism has suffered from the following disadvantages and drawbacks. Namely, timing error between the operation of the solenoid and the control wheel is apt to occur because of the variations in the predetermined intervals, and because of variations in the rotational speed of the control wheel. In detail, each of the predetermined intervals is defined by a time constant circuit having resistor(s) and capacitor(s), and it is difficult to obtain a uniform time constant throughout a number of cirucits because of variations in resistances and capacitances. In addition, when a D.C. motor is used for driving the control wheel, the rotational speed of the control wheel is apt to vary as the amount of load varies. The above-mentioned timing error corresponds to the product of the error of the predetermined interval and the error of the rotational speed of the control wheel.
The timing error between the operating period of the solenoid and rotational angle of the control wheel results in malfunction of the operating mode switching mechanism, and in the worst case, changeover between operating modes cannot be performed.